Access a blender file externally (from a python script)
Only with Phyton isn't it possible I think, but with the BatchBlendChanger  Add-on it is.
You can download the Add-on here:
https://github.com/RivinHD/BatchBlendChanger
Explanation:

Start Blender and install the Add-on.
Activate the Add-on.
Search in the Search Menu (Shortcut: F3) for &quot;Batch Import&quot;.
Now you should see a file selector.
Select all .blend files you want to change the path to relative.
Search in the Search Menu &quot;Batch Start&quot;.
A Dialog box/Popup should appear with all you selected .blend files.
Press OK to start the process.

The process is running and could take a while with 150 files.
If Blender crashes, no problem, restart Blender again. The Process will run again at exactly the same position it was running before the crash.
You should you use the console to start blender. It's easier to restart with the console and you can watch the process.
Start Blender with the console:

open the console (cmd)
Navigate to the installation path of Blender
Start blender with the command &quot;blender.exe -d&quot;